The one-gene, one-enzyme relationship has undergone several modifications in light of our current knowledge of molecular biology. Many proteins, including many enzymes, are composed of more than one *polypeptide chain, or subunit. So it is more correct to speak of a one-gene, one-polypeptide relationship.

So far we have seen that in terms of protein synthesis, the function of a gene is to prescribe the production of a single, specific polypeptide. But not all genes code for polypeptides. As we will see below and in Chapter 16, many DNA sequences are transcribed to RNA molecules that are not translated into polypeptides, but instead have other functions.